Method for producing isocyanates

ABSTRACT

In a process for preparing isocyanates by reacting the corresponding primary amines with phosgene in an inert solvent, use is made of from 0.01 to 50 mol % of a sulfoxide, a sulfone or a mixture thereof, based on the total amount of primary amine and isocyanate formed in the reaction solution.

The present invention relates to an improved process for preparingisocyanates by reacting the corresponding primary amines with phosgenein an inert solvent.

Isocyanates are large-scale industrial products having many uses in thefield of polyurethane polymers. However, some isocyanates are alsoemployed in the preparation of pharmaceutically active compounds.

The synthesis of isocyanates by reaction of amines with phosgene hasbeen known for a long time. Two basic processes are described in theliterature, of which one is carried out at atmospheric pressure and theother is carried out under superatmospheric pressure. The phosgenationunder superatmospheric pressure has the disadvantage that it requires agreatly increased outlay in terms of apparatus to cope with theincreased safety risk, namely the escape of phosgene.

A process for preparing sulfonyl isocyanates at atmospheric pressure, inwhich a solution of a sulfonyl amide and an isocyanate as catalyst isreacted with phosgene in an inert solvent, is known from U.S. Pat. Nos.3,371,114 and 3,484,466. In this process, the corresponding sulfonylureais formed as an intermediate and reacts with phosgene to form thedesired sulfonyl isocyanate.

Alkyl and aryl isocyanates are usually prepared from the correspondingamines in two stages at atmospheric pressure by the phosgenationprocesses described, for example, in Ullmann's Encyclopedia ofIndustrial Chemistry, 6^(th) edition, 2000 electronic release, Chapter“ISOCYANATES, ORGANIC—Production”. In the first stage, the coldphosgenation, the amine is reacted with an excess of phosgene in highlydilute solution and reacted at low temperatures to form thecorresponding carbamoyl chloride from which the isocyanate is formed inthe second stage at elevated temperature, viz. the hot phosgenation.Owing to their increased basicity compared to aromatic amines, aliphaticand cycloaliphatic primary amines are more difficult to phosgenate andlead to increased formation of by-products. Disadvantages of theseprocesses are the need to carry out the phosgenation in two stages andalso, in particular, the formation of an intermediate suspension ofsparingly soluble carbamoyl chloride and amine hydrochloride which inturn makes increased dilution of the reaction medium necessary toprevent deposits on and blockages of plant components. Owing to thesolid formed, this process cannot be carried out continuously atatmospheric pressure. Furthermore, the process results in formation of asymmetrically N,N′-disubstituted urea as by-product, and the formationof this can only be suppressed at the cost of drastically reducedspace-time yields.

Aliphatic and cycloaliphatic amines are frequently used in the form oftheir salts in the cold/hot phosgenation. However, these salts aresparingly soluble in the reaction medium, so that additional reactionsteps and very long reaction times become necessary.

Furthermore, U.S. Pat. No. 3,440,268 and U.S. Pat. No. 3,492,331 teachthe reaction of primary amines with phosgene in the presence of anN,N-disubstituted formamide, an N-alkyllactam, an N,N-disubstitutedN′-arylformamidine or an N,N,N′,N′-tetrasubstituted N″-arylguanidine ascatalyst. H. Ulrich, Chemistry & Technology of Isocyanates, Wiley &Sons, 1996, pages 328 to 330, discloses the reaction of primary amineswith phosgene in the presence of tertiary amines, tetramethylurea andcarbonyldiimidazole as catalyst, and CS 262 295 discloses the use ofN,N′-diazabicyclo[2.2.2]octane as catalyst. Some of the compoundsspecified have to be used in equimolar amounts and form sparinglysoluble salts in the form of the catalyst hydrochloride adducts underthe reaction conditions. Furthermore, the amine used and the hydrogenchloride formed react to produce sparingly soluble amine hydrochloride.

WO 01/17951 teaches the preparation of isocyanates by phosgenation ofthe corresponding primary amines in the presence of a monoisocyanatewhich is initially charged in an inert solvent prior to commencement ofthe reaction and is admixed with phosgene. A disadvantage of thisprocess is that the preferred low molecular weight aliphatic isocyanatesare toxic and require strict safety precautions.

It is an object of the present invention to find a process for preparingisocyanates which no longer has the above mentioned disadvantages, canbe carried out both continuously and batchwise, displays no formation oronly insignificant formation of sparingly soluble components, can becarried out without a highly toxic catalyst, makes it possible for thereaction to be carried out in only one reaction stage and leads to ahigh conversion, a high selectivity and a high space-time yield evenunder mild temperature and pressure conditions.

We have found that this object is achieved by a process for preparingisocyanates by reacting the corresponding primary amines with phosgenein an inert solvent, wherein from 0.01 to 50 mol % of a sulfoxide, asulfone or a mixture thereof, based on the total amount of primary amineand isocyanate formed in the reaction solution, is used.

The sulfoxide which can be used in the process of the present inventionhas the formula (I)R¹—S(O)—R²  (I)and the sulfone which can be used in the process of the presentinvention has the formula (II)R¹—S(O)₂—R²  (II),where the radicals R¹ and R² are each, independently of one another, acarbon-containing organic radical and may also be joined to one another.

For the purpose of the present invention, a carbon-containing organicradical is an unsubstituted or substituted, aliphatic, aromatic oraraliphatic radical. This may contain one or more heteroatoms such asoxygen, nitrogen, sulfur or phosphorus, for example —O—, —S—, —SO—,—SO₂—, —NR—, —CO—, —N═, —SiR₂—, —PR—and/or —PR₂, and/or be substitutedby one or more functional groups which contain, for example, oxygen,nitrogen, sulfur and/or halogen, for example by fluorine, chlorine,bromine, iodine and/or a cyano group (the radical R is likewise acarbon-containing organic radical). The carbon-containing organicradical can be a monovalent radical or else a divalent or trivalentradical.

Monovalent radicals R¹ and R² are each preferably, independently of oneanother,

-   -   an unbranched or branched, acyclic or cyclic, unsubstituted or        substituted alkyl radical which has from 1 to 30 aliphatic        carbon atoms and in which one or more of the CH₂ groups may be        replaced by heteroatoms such as —O— or —S— or by        heteroatom-containing groups such as —CO—, —SO—, —SO₂—, —NR— or        —SiR₂—, and in which one or more of the hydrogen atoms may be        replaced by substituents such as aryl groups or functional        groups; or    -   an unsubstituted or substituted aromatic radical which has from        3 to 30 carbon atoms and one ring or two or three fused rings        and in which one or more ring atoms may be replaced by        heteroatoms such as nitrogen and in which one or more of the        hydrogen atoms may be replaced by substituents such as alkyl or        aryl groups or functional groups.

Divalent radicals made up of R¹ together with R² are preferably each

-   -   an unbranched or branched, acyclic or cyclic, unsubstituted or        substituted C₄–C₂₀-alkylene radical (“divalent alkyl radical”)        which has from 4 to 10 atoms in the alkylene chain and in which        CH₂ groups may be replaced by hetero groups such as —CO—, —O—,        —S—, —SO—, —SO₂—, —SiR₂— or —NR— and in which one or more of the        hydrogen atoms may be replaced by substituents such as aryl        groups.

Particular preference is given to using sulfoxides (I) and/or sulfones(II) whose radicals R¹ and R² are each, independently of one another,

-   -   an unbranched or branched C₁–C₂₀-alkyl radical such as methyl,        ethyl, 1-propyl, 2-propyl (sec-propyl), 1-butyl, 2-butyl        (sec-butyl), 2-methyl-1-propyl (isobutyl), 2-methyl-2-propyl        (tert-butyl), 1-pentyl, 2-pentyl, 3-pentyl, 2-methyl-2-butyl        (tert-amyl), 1-hexyl, 2-hexyl, 3-hexyl, 2-methyl-2-pentyl,        3-methyl-3-pentyl or 2-methoxy-2-propyl;    -   an unbranched or branched C₅–C₂₀-cycloalkyl radical such as        cyclopentyl, cyclohexyl or cyclooctyl; or    -   a C₆–C₂₀-aryl or C₃–C₂₀-heteroaryl radical which may be        unsubstituted or substituted by one or more C₁–C₄-alkyl        radicals, for example phenyl, 2-methylphenyl (o-tolyl),        3-methylphenyl (m-tolyl), 4-methylphenyl (p-tolyl),        2,6-dimethylphenyl, 2,4-dimethylphenyl, 2,4,6-trimethylphenyl,        2-methoxyphenyl, 3-methoxyphenyl, 4-methoxyphenyl, 2-pyridyl,        3-pyridyl, 4-pyridyl, 3-pyridazinyl, 4-pyridazinyl,        5-pyridazinyl, 2-pyrimidinyl, 4-pyrimidinyl, 5-pyrimidinyl,        2-pyrazinyl, 2-(1,3,5-triazin)yl, 1-naphthyl, 2-naphthyl,        2-quinolyl, 8-quinolyl, 1-isoquinolyl or 8-isoquinolyl.

Very particular preference is given to using sulfoxides (I) and/orsulfones (II) whose radicals R¹ and R² are each, independently of oneanother,

-   -   an unbranched or branched C₁–C₁₀-alkyl radical;    -   an unbranched or branched C₅–C₁₀-cycloalkyl radical; or    -   a C₆–C₁₂-aryl radical which may be unsubstituted or substituted        by one or more C₁–C₄-alkyl radicals.

In particular, dimethyl sulfoxide or dimethyl sulfone is used in theprocess of the present invention.

The sulfoxide, the sulfone or the mixture thereof is used in a catalyticamount of from 0.01 to 50 mol %, based on the total amount of primaryamine and isocyanate formed in the reaction solution. In calculating thetotal amount of primary amine and isocyanate formed, the molar amountsof the not yet reacted starting material (primary amine) and the productwhich has been formed (isocyanate) and any intermediates present are tobe added. In the process of the present invention, preference is givento using from 0.01 to 25 mol %, particularly preferably from 0.5 to 20mol % and very particularly preferably from 1 to 15 mol %, of sulfoxide,sulfone or a mixture thereof, based on the total amount of primary amineand isocyanate formed in the reaction solution.

In the process of the present invention, the sulfoxide, the sulfone orthe mixture thereof is generally initially charged in an inert solvent.For the purposes of the present invention, inert solvents are solventswhich are chemically inert toward the primary amine used, the phosgene,the isocyanate formed and the sulfoxide or sulfone used. “Chemicallyinert” means that the diluents do not react chemically with thesubstances mentioned under the chosen conditions. Preference is given tousing aromatic or aliphatic hydrocarbons, particularly preferablymonosubstituted or polysubstituted aromatic hydrocarbons such astoluene, o-, m-, p-xylene, ethylbenzene, chlorobenzene or o-, m-,p-dichlorobenzene. Very particular preference is given to o-, m- orp-xylene, chlorobenzene, o-, m-, p-dichlorobenzene and mixtures thereof.

In general, the introduction of phosgene is then commenced. It can beintroduced in liquid or gaseous form. It is usual for from about 10 to50% of the theoretically required amount of phosgene, based on thereaction volume, to be introduced initially.

Addition of the primary amine to the phosgene-laden starting solution ofsulfoxide, sulfone or a mixture thereof in the inert solvent is thencommenced. Further phosgene is introduced in accordance with theprogress of the reaction and the amount of amine added.

The primary amines which can be used in the process of the presentinvention have the formula (II)R³—NH₂  (II),where the radical R³ is a carbon-containing organic radical as definedabove.

The radical R³ is preferably

-   -   an unbranched or branched, acyclic or cyclic, unsubstituted or        substituted alkyl radical which has from 1 to 30 aliphatic        carbon atoms and in which one or more of the CH₂ groups may be        replaced by heteroatoms such as —O—, or —S— or by        heteroatom-containing groups such as —CO—, —NR— or —SiR₂— and in        which one or more of the hydrogen atoms may be replaced by        substituents such as aryl groups or functional groups, with the        exception of —OH, —SH and —COOH groups; or    -   an unsubstituted or substituted aromatic radical which has 3 to        30 carbon atoms and one ring or two or three fused rings and in        which one or more ring atoms may be replaced by heteroatoms such        as nitrogen and in which one or more of the hydrogen atoms may        be replaced by substituents such as alkyl or aryl groups or        functional groups, with the exception of —OH, —SH and —COOH        groups.

In particular, the radical R³ may bear one or more further NH₂ groups,so that oligoamines having two or more NH₂ groups are explicitlyincluded as primary amines. The primary amine used is particularlypreferably a monoamine having one NH₂ group or a diamine having two NH₂groups, each having from 1 to 20 carbon atoms.

As examples of suitable acyclic and substituted or unsubstituted alkylradicals R³, mention may be made of methyl, ethyl, 1-propyl, 2-propyl(sec-propyl), 1-butyl, 2-butyl (sec-butyl), 2-methyl-1-propyl(isobutyl), 2-methyl-2-propyl (tert-butyl), 1-pentyl, 2-pentyl,3-pentyl, 2-methyl-2-butyl (tert-amyl), 1-hexyl, 2-hexyl, 3-hexyl,2-methyl-2-pentyl, 3-methyl-3-pentyl, 1-octyl, 1-decyl, 2-aminoethyl,3-aminopropyl, 4-aminobutyl, 5-aminopentyl, 6-aminohexyl, 8-aminooctyl,phenylmethyl, 1-phenylethyl, 1-phenylpropyl or 1-phenylbutyl.

As examples of cyclic and substituted or unsubstituted cycloalkylradicals R³, mention may be made of cyclopentyl, cyclohexyl, cyclooctylor 3-aminomethyl-3,5,5-trimethylcyclohexyl.

As examples of aromatic and heteroaromatic and substituted orunsubstituted radicals R³, mention may be made of phenyl, o-, m-,p-tolyl and aminotolyls.

It may be emphasized that enantiomerically pure, optically activecompounds or mixtures thereof having an optically active carbon bound tothe NH₂ group can also be used as primary amines.

Advantageously, the process of the present invention leads to only asmall degree of racemization, generally below 2%, during the reactionand the work-up.

The primary amine used in the process of the present invention is veryparticularly preferably 1,6-diaminohexane, cyclohexylamine,isophoronediamine (3-aminomethyl-3,5,5-trimethylcyclohexylamine),aniline, a diaminotoluene, diphenylmethane-4,4′-diamine, R-(+)- andS(−)-phenylethylamine.

The molar ratio of the total amount of phosgene introduced to theprimary amine groups to be phosgenated is generally from 1 to 10,preferably from 1 to 5 and particularly preferably from 1 to 2, inparticular from 1.2 to 1.8.

The amount of inert solvent is generally from 100 to 2000% by weight andpreferably from 300 to 1000% by weight, based on the total amount of theprimary amine present and the isocyanate formed.

The process of the present invention is generally carried out at from 20to 200° C., preferably from 20 to 150° C. and particularly preferablyfrom 50 to 100° C. In carrying out the process of the present invention,the pressure is generally from 0.05 to 0.5 MPa abs and preferably from0.08 to 0.12 MPa abs.

After the desired amount of phosgene and amine has been added, thereaction solution obtained is generally left for a certain time, ingeneral from 30 minutes to 6 hours, under the reaction conditions toallow further reaction to occur. To remove or reduce the amount ofexcess phosgene and its reaction products carbon dioxide and hydrogenchloride from/in the reaction solution, it is usual for inert gassubsequently to be passed through the mixture with intensive mixing(“stripping”).

The process of the present invention can in principle be carried outeither continuously or batchwise, with continuous operation beingpreferred. The process can be carried out in any apparatus suitable fora reaction with phosgene. Suitable reactors are, for example, stirredvessels.

The reaction mixture after the reaction is generally worked up by knownmethods. Preference is given to isolating the desired isocyanate byfractional distillation. The sulfoxide, sulfone or mixture thereof usedas catalyst is preferably recovered by distillation and, in the case ofa continuous process, recirculated.

In a general embodiment for the batchwise preparation of isocyanates,the sulfoxide, the sulfone or the mixture thereof together with an inertsolvent are placed while stirring in a reactor, for example a stirredvessel, and the solution is loaded with phosgene. The reaction system isthen brought to the desired temperature and the introduction of theamine is commenced. Further phosgene is introduced in accordance withthe progress of the reaction and the amount of amine fed in. After thedesired amount of phosgene has been fed in, the reaction solution isleft at the set temperature for some time while continuing to stir toallow further reaction to occur. During this after-reaction time,phosgene which is still present in the reaction solution reacts withresidual amine. To remove or reduce the concentration of the excessphosgene and its reaction products carbon dioxide and hydrogen chloridefrom/in the reaction solution, it is possible to pass inert gas throughthe mixture while mixing intensively (“stripping”). The reactionsolution obtained is then passed to work-up. In general, the work-up iscarried out by distillation, if appropriate under reduced pressure. Inthe case of relatively high-boiling isocyanates, other purificationmethods, for example crystallization, are also possible.

In a general embodiment for the continuous preparation of isocyanates,the sulfoxide, the sulfone or the mixture thereof together with an inertsolvent are placed while stirring in the reactor, for example a stirredvessel, and the solution is loaded with phosgene. The reaction system isthen brought to the desired temperature and the continuous introductionof the amine is commenced. Further phosgene is introduced continuouslyin accordance with the progress of the reaction and the amount of aminefed in. After the contents of the reactor have largely reacted to formisocyanate, the amounts of amine and phosgene are adjusted so that bothare added essentially in the stoichiometric required ratio. An amount ofthe reaction solution corresponding to the amount fed in is taken fromthe reaction apparatus, for example via a level regulator or anoverflow. The reaction solution which has been taken off is collected ina downstream container, for example a stirred vessel, to allow furtherreaction to occur. After the downstream container has been filled by thereaction mixture, the overflow is, if appropriate, freed of thecoproducts carbon dioxide and hydrogen chloride by stripping asdescribed above and is passed to work-up. The work-up can be carriedout, for example, by distillation.

The process of the present invention makes it possible to prepareisocyanates by continuous or batchwise phosgenation of primary amines.Compared to known, catalyst-free processes, it makes it possible tocarry out the actual reaction in only a single reaction stage under mildtemperature and pressure conditions, and also gives a higher conversionof primary amine, a high selectivity and a high space-time yield ofisocyanate. Compared to the known, catalyst-free processes and the knownprocesses in the presence of a catalyst, the process of the presentinvention has no tendency, or only an insignificant tendency, to formsparingly soluble components. The risk of deposits on and blockages ofplant components is significantly reduced thereby, which represents adecisive safety advantage when handling toxic phosgene. Furthermore, thenonformation or only insignificant formation of sparingly solublecomponents makes it possible to carry out a continuous process for thefirst time and gives significant advantages in the subsequent work-up.

The sulfoxides and sulfones used in the process of the present inventionare generally nontoxic or have only a low toxicity, which represents, inparticular, an advantage over the use of an aliphatic monoisocyanatedescribed in WO 01/17951.

EXAMPLES

Experimental Apparatus

The experimental apparatus comprised a 1 l glass vessel provided with astirrer, a thermostat, an inlet tube for the gaseous phosgene and atwo-part condenser cascade. The two-part condenser cascade comprises anintensive condenser which was maintained at −10° C. and a carbon dioxidecondenser which was maintained at −78° C. The experiments were carriedout at atmospheric pressure.

Comparative Example 1

500 g of chlorobenzene were placed in the glass vessel and 40 g ofgaseous phosgene was introduced at room temperature. The reactionmixture was subsequently heated to 77° C., with vigorous phosgene refluxbeing established. While stirring vigorously at 77–80° C., a total of99.2 g of cyclohexylamine (1 mol) dissolved in 200 g of chlorobenzeneand at the same time a further 92 g of phosgene were introduced over aperiod of 3 hours. After addition was complete, the system wasmaintained at 77–80° C. for a further one hour without introduction ofphosgene to allow further reaction to occur and the residual unreactedphosgene was subsequently stripped out at 50° C. by means of nitrogen.The reaction mixture obtained was a suspension from which the solidobtained was separated by filtration. 15 g of solid which, according toIR-spectroscopic analysis, was mainly amine hydrochloride were able tobe isolated in this way. The filtered crude product was worked up bydistillation. Removal of the solvent and fractional distillation gave88.8 g of cyclohexyl isocyanate (0.710 mol). This corresponds to 71.0%of the theoretical yield.

Example 2 According to the Present Invention

500 g of chlorobenzene and 2 g of dimethyl sulfoxide (0.025 mol) wereplaced in the glass vessel and 35 g of gaseous phosgene was introducedat room temperature. The reaction mixture was subsequently heated to 78°C., with vigorous phosgene reflux being established. While stirringvigorously at 78–82° C., a total of 99.2 g of cyclohexylamine (1 mol)dissolved in 200 g of chlorobenzene and at the same time a further 113 gof phosgene were introduced over a period of 3 hours. After addition wascomplete, the system was maintained at 78–82° C. for a further one hourwithout introduction of phosgene to allow further reaction to occur andthe residual unreacted phosgene was subsequently stripped out at 40° C.by means of nitrogen. The reaction mixture obtained was a fluidsuspension from which the solid obtained was separated by filtration.2.2 g of solid were able to be isolated in this way. The filtered crudeproduct was worked up by distillation. Removal of the solvent andfractional distillation gave 79.7 g of cyclohexyl isocyanate (0.631mol). This corresponds to 63.1% of the theoretical yield.

Example 3 According to the Present Invention

500 g of chlorobenzene and 2.4 g of dimethyl sulfone (0.025 mol) wereplaced in the glass vessel and 30 g of gaseous phosgene was introducedat room temperature. The reaction mixture was subsequently heated to 78°C., with vigorous phosgene reflux being established. While stirringvigorously at 78–80° C., a total of 99.2 g of cyclohexylamine (1 mol)dissolved in 200 g of chlorobenzene and at the same time a further 105 gof phosgene were introduced over a period of 3 hours. After addition wascomplete, the system was maintained at 78–80° C. for a further one hourwithout introduction of phosgene to allow further reaction to occur andthe residual unreacted phosgene was subsequently stripped out at 50° C.by means of nitrogen. The reaction mixture obtained was a suspensionfrom which the solid obtained was separated by filtration. 7.8 g ofsolid were able to be isolated in this way. The filtered crude productwas worked up by distillation. Removal of the solvent and fractionaldistillation gave 86.8 g of cyclohexyl isocyanate (0.694 mol). Thiscorresponds to 69.4% of the theoretical yield.

Compared to comparative example 1 without use of a catalyst, example 2according to the present invention using dimethyl sulfoxide displays aproportion of solids which is reduced by a factor of 7 and example 3according to the present invention using dimethyl sulfone displays aproportion of solids which is reduced by a factor of 2. Thus,significantly less solid has to be separated off.

The lower proportion of solids enables the process of the presentinvention to be carried out significantly more simply, more safely andwith fewer problems. In particular, the risk of deposits on andblockages of plant components is significantly reduced, which representsa decisive safety advantage when handling toxic phosgene and makes acontinuous process possible.

Compared to the known processes which have to be carried out batchwisebecause of solids formation, the ability to carry out a continuousprocess enables a significant increase in the space-time yield to beachieved. Unreacted amine can be returned to the reaction apparatus, sothat the achievable yield is also higher than in the known processes.

1. A process for preparing isocyanates which comprises reacting thecorresponding primary amines with phosgene in an inert solvent in thepresence of from 0.01 to 50 mol % of a sulfoxide, a sulfone or a mixturethereof, based on the total amount of primary amine and isocyanateformed in the reaction solution.
 2. A process as claimed in claim 1,wherein from 0.01 to 25 mol % of the sulfoxide, the sulfone or themixture thereof, based on the total amount of primary amine andisocyanate formed in the reaction solution, is used.
 3. A process asclaimed in claim 1, wherein a sulfoxide of the formula (I)R¹—S(O)—R²  (1) is used as sulfoxide and/or a sulfone of the formula(II)R¹—S(O)₂—R²  (II), is used as sulfone, where the radicals R¹ and R² areeach, independently of one another, an unbranched or branchedC₁–C₁₀-alkyl radical, an unbranched or branched C₅–C₁₀-cycloalkylradical or a C₆–C₁₂-aryl radical which may be unsubstituted orsubstituted by one or more C₁–C₄-alkyl radicals.
 4. A process as claimedin claim 3, wherein dimethyl-sulfoxide or dimethyl sulfone is used.
 5. Aprocess as claimed in claim 1, wherein the primary amine used is amonoamine having one NH₂ group or a diamine having two NH₂ groups, eachhaving from 1 to 20 carbon atoms.
 6. A process as claimed in claim 5,wherein the primary amine used is 1,6-diaminohexane, cyclohexylamine,isophoronediamine, aniline, a diaminotoluene,diphenylmethane-4,4′-diamine, R-(+)- or S(−)-phenylethylamine.
 7. Aprocess as claimed in claim 1, wherein the reaction is carried out atfrom 20 to 200° C. and a pressure of from 0.05 to 0.5 MPa abs.
 8. Aprocess as claimed in claim 1, wherein the inert solvent is used in anamount of from 100 to 2000% by weight, based on the total amount ofprimary amine present and isocyanate formed.
 9. A process as claimed inclaim 1, wherein the inert solvent used is o-, m- or p-xylene,chlorobenzene, o-, m-, p-dichlorobenzene or a mixture thereof.